1. Field of the Invention
The present invention relates to a head substrate, printhead, head cartridge, and printing apparatus. Particularly, the present invention relates to a head substrate prepared by forming, on the same substrate, an electrothermal transducer for generating heat energy necessary to print, and a driver circuit for driving the electrothermal transducer, a printhead using the head substrate, a head cartridge using the printhead, and a printing apparatus.
2. Description of the Related Art
The electrothermal transducers (heaters) and driver circuits of a printhead mounted in a conventional inkjet printing apparatus are formed on the same substrate by a semiconductor process technique as disclosed in, for example, U.S. Pat. No. 6,290,334. There has already been proposed a substrate on which an ink supply channel for supplying ink is arranged on the substrate and heaters are arrayed at positions opposite to each other near the ink supply channel.
FIG. 6 is a view showing the layout of a head substrate used in a conventional inkjet printhead. As a method of driving a printhead of this type, time-divisional driving is put into practical use. A maximum power capable of simultaneously driving heaters has an upper limit. According to the time-divisional driving, a plurality of heaters are divided into M heater blocks each of N heaters, and N heaters of each heater block are simultaneously driven.
In FIG. 6, a substrate 100 is formed by integrating, by a semiconductor process technique, heaters and driver circuits for driving them. A heater & driver array 101 is an array of heaters and drivers. The driver includes a driver transistor which serves as a driving element. An ink supply channel 102 supplies ink from the back surface of the substrate. Each shift register (S/R) 103 temporarily stores print data. Each latch circuit 104 latches print data stored in the corresponding shift register (S/R) 103 at once. Each decoder 105 selects a desired heater block of the heater & driver array 101. Each input circuit block 106 includes a buffer circuit for inputting digital signals to the shift register 103 and decoder 105. The decoder receives a block selection signal as a control signal. Signal lines 107 transmit signals from the shift register 103 and decoder 105 to select individual segments in the heater & driver array 101. Each contact pad 110 is used to input/output an electrical signal from/to outside the substrate.
FIG. 7 is a circuit diagram showing an equivalent circuit corresponding to one segment (one heater) of the heater & driver array 101 which are integrated on the head substrate shown in FIG. 6 and drive heaters for discharging ink.
In a head substrate layout as shown in FIG. 6, the contact pads 110, input circuit blocks 106, decoders 105, shift registers 103, and latch circuits 104 are arranged at the ends of the head substrate 100 in a longer side direction. In this layout, the signal lines 107 are provided along the longer side direction of the head substrate 100.
In FIG. 7, an AND circuit 201 calculates the logical product of two input signals. The AND circuit 201 receives a block selection signal which is sent from the decoder 105 to select heaters of each block, and a print data signal which is transferred to the shift register 103 and latched by the latch circuit 104. Based on the logical product, each segment can be selectively turned on. An inverter circuit 202 buffers an output from the AND circuit 201. A VDD power supply line 203 serves as the power supply of the inverter circuit 202. An inverter circuit 204 buffers an output from the inverter circuit 202. A VH power supply line 205 is used for supplying a voltage to be applied to a heater. A driver transistor 207 serves as a switching element for switching between supplying a current and not supplying the current, to a heater 206. A VHTM power supply line 208 serves as a power supply for supplying power to the inverter circuit 204 functioning as a buffer, thereby applying a gate voltage to the driver transistor 207. A voltage conversion circuit 209 converts the voltage of an output signal from the AND circuit 201 into a voltage VHTM for driving the driver transistor 207. The voltage conversion circuit 209 incorporates a level converter 210 which converts a voltage to the voltage VHTM.
FIG. 8 is an equivalent circuit diagram of a circuit corresponding to one bit of the shift register 103 and latch circuit 104 which temporarily store print data.
In FIG. 8, print data DATA is input to the shift register in synchronism with a clock CLK, and the input print data is latched in synchronism with a latch signal LT. When a heat enable signal HE is input, a print data signal is output from the latch circuit to the AND circuit 201 while the heat enable signal is enabled.
FIG. 9 is a timing chart for explaining a series of operations from receiving print data in the shift register 103 to driving the heater 206 by supplying a current to it.
In FIG. 9, print data is supplied to a data pad (not shown) in synchronism with the clock CLK input to a clock pad (not shown). The shift register 103 temporarily stores the print data. The latch circuit 104 latches the print data in synchronism with the latch signal LT supplied to a latch pad (not shown). Then, the logical product of a block selection signal for selecting heaters of a desired block, and a print data signal held in accordance with the latch signal LT is calculated. A heater current (current VH) flows in synchronism with the heat enable signal HE, which directly determines a current driving time, and the logical product.
Printing is performed by repeating the series of operations for respective blocks.
FIG. 10 is a view showing connection of power supply wiring lines in the head substrate shown in FIG. 6.
In FIG. 10, power supply pads VH 130, 132, 134, and 136 supply voltages to be applied to heaters. Ground pads GND 131, 133, 135, and 137 correspond to the power supply pads. Wiring lines 140 are divided to independently supply power from the power supply pads VH to respective blocks. Wiring lines 141 are divided to feed back power from the blocks to the ground pads GND. These wiring lines will be called VH power supply wiring lines and GND wiring lines.
Segments including heaters and driver transistors arranged on the head substrate are divided into 16 groups A to P. Power is independently supplied and fed back to and from each group in order to keep power loss constant by making uniform the wiring resistances of the VH power supply wiring lines and GND wiring lines which are connected to the respective groups. The widths of the wiring lines are adjusted to have the same resistance value. Each group is comprised of segments (including heaters), respectively belonging to different time-divisionally driven blocks.
A head substrate on which ink supply channel arrays are staggered is proposed in, e.g., Japanese Patent Publication Laid-Open No. 2006-88648.
However, according to the power supply wiring connection as shown in FIG. 10, the wiring becomes longer as the longer side of the chip (head substrate) becomes longer. In addition, as the group division count increases, the widths of wiring lines independently connected to respective groups become narrower, and the wiring resistance tends to rise as a whole. The increase in wiring resistance causes so-called power loss because power, which should be originally consumed by heaters, is consumed by the wiring to a certain degree. If the original power supply voltage is increased to compensate for the power loss, this adversely affects the durable service life of heaters. Further, heat generated by power consumption by the wiring raises the temperature of the printhead itself, adversely affecting the ink discharge characteristic.
As for a head substrate on which ink supply channel arrays are staggered, the above reference (Japanese Patent Publication Laid-Open No. 2006-88648) does not disclose a specific layout of circuits on the head substrate. A circuit layout effectively utilizing a head substrate with a limited area is required.